1. Field of the Invention
The invention relates to a system and method for controlling multiple camshafts in a variable cam timing engine.
2. Background of the Invention
Engines have utilized variable cam timing (VCT) mechanisms to control the opening and closing of intake valves and exhaust valves communicating with engine cylinders. In particular, each VCT mechanism is usually utilized to adjust a position of a camshaft (which actuates either intake valves or exhaust valves or both) with respect to a crankshaft position. By varying the position of the camshaft (i.e., camshaft angle) with respect to the position of the crankshaft, engine fuel economy can be increased and engine emissions can be decreased.
In these engines having VCT mechanisms, the inventors of the present invention have realized that it is desired to shift the position of camshafts in the VCT mechanisms synchronously (i.e., at the same speed) to a desired phase angle with respect to the crankshaft. However, the inventors herein have also recognized that first and second camshafts associated with first and second VCT mechanisms, respectively, in an engine, may not follow the same trajectory, or move at different rates, to the desired phase angle. For example, the first VCT mechanism may be actuated at a lower pressure than a second VCT mechanism due to a clogged oil line communicating with the first VCT, resulting in different movement of the first camshaft. Still further, the first VCT mechanism may “stick” at cold temperatures resulting in different movement of the first camshaft as compared to the second camshaft of the second VCT mechanism. During non-synchronous movement of the first and second camshafts, the air charge delivered to first and second cylinder banks, respectively, may be different. The difference in air charge can result in differing torques being produced by the first and second cylinder banks resulting in undesirable engine shaking and increased engine noise. Further, the difference in air charge may result in non-optimal spark timing in one of the cylinder banks resulting in increased engine knock in the cylinder bank. Still further, the difference in air charge may result in a rich air-fuel mixture being delivered to one of the cylinder banks resulting in decreased fuel economy.
One approach for controlling engines with multiple cam timing actuators attempts to synchronize the cam operation based on determining which actuator has the slowest response. In particular, after the slowest actuator is detected, the other actuators are slaved to the slowest actuator, thereby attempting to keep all the actuators moving at approximately the same speed. Such a method is described in U.S. application Ser. No. 10/036,045, filed Nov. 9, 2001, “System and Method for Controlling Dual Camshafts in a Variable Cam Timing Engine”, which has been assigned to the assignee of the present invention.
The inventors herein have recognized a disadvantage with the above approach. In particular, while slowing down faster actuators may result in all actuators moving with approximately the same velocity, this does not necessarily provide synchronized movement. In other words, different cam shafts may be moving at the same velocity, but in different positions. Thus, while the velocities may be synchronized, since cam position affects engine breathing, ignition timing, and various other parameters, this parameter can be more important than cam velocity.